1. Field
The present embodiments relate to an organic light emitting diode display and a driving method thereof.
2. Related Art
Recently, flat panel display devices have been reduced in weight and bulk and are capable of eliminating some of the disadvantages of a cathode ray tube (CRT). Flat panel display devices include, for example, a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP) and an electro-luminescence (EL) display.
The EL display in such display devices is a self-luminous device capable of light-emitting a phosphorous material by a re-combination of electrons with holes. The EL display device is generally classified into an inorganic EL device using an inorganic compound as the phosphorous material and an organic EL device using an organic compound as the phosphorous material. Such an EL display device has many advantages of, for example, a low voltage driving, a self-luminescence, a thin-thickness, a wide viewing angle, or a fast response speed and a high contrast such that it can be highlighted into a post-generation display device.
The organic EL device is usually comprised of an electron injection layer, an electron carrier layer, a light-emitting layer, a hole carrier layer and a hole injection layer that are disposed between a cathode and an anode. In such an organic EL device, when a predetermined voltage is applied between the anode and the cathode, electrons produced from the cathode are moved, via the electron injection layer and the electron carrier layer, into the light-emitting layer. Holes produced from the anode are moved, via the hole injection layer and the hole carrier layer, into the light-emitting layer. Thus, the electrons and the holes fed from the electron carrier layer and the hole carrier layer emit a light by their re-combination at the light-emitting layer.
Referring to FIG. 1, an active matrix type EL display device using an organic EL device includes an EL panel 20 having pixels 28 arranged at each area defined by intersections between scan lines SL and data lines DL. A scan driver 22 drives the scanning lines SL of the EL panel 20. A data driver 24 drives the data lines DL of the EL panel 20. A gamma voltage generator 26 supplies a plurality of gamma voltages to the data driver 24. A timing controller 27 controls the data driver 24 and the scan driver 22. A power supply 15 supplies a power voltage to the pixels 28.
The EL panel 20 has the pixels arranged in a matrix type. The EL panel 20 is provided with a supply pad 10 that receives a supply voltage VDD from the power supply 15 and a ground pad 12 that receives a ground voltage GND from the power supply 15. The supply voltage VDD supplied to the supply pad 10 is applied to each pixel 28. The ground voltage GND supplied to the ground pad 12 also is applied to each pixel 28.
The scan driver 22 applies a scanning pulse to the scan lines SL to sequentially drive the scan lines SL.
The gamma voltage generator 26 supplies gamma voltage having various voltage values to the data driver 24.
The data driver 24 converts digital data signals inputted from the timing controller 27 into analog data signals with the aid of gamma voltages from the gamma voltage generator 26. The data driver 24 applies the analog data signals to the data lines DL whenever the scanning pulse is supplied.
The timing controller 27 generates a data control signal for controlling the data driver 24 and a scanning control signal for controlling the scan driver 22 with the aid of synchronizing signals supplied from an external system, for example, a graphic card. The data control signal generated from the timing controller 27 is applied to the data driver 24 to thereby control the data driver 24. The scanning control signal generated from the timing controller 27 is applied to the scan driver 22 to control the scan driver 22. The timing controller 27 applies the digital data signals from the external system to the data driver 24.
Each of the pixels 28 receives the data signal from the data line DL when the scanning pulse is applied to the scan line SL, to generate a light that corresponds to the data signal.
A detailed configuration of the pixel 28 is shown in FIG. 2. Referring to FIG. 2, the pixel 28 includes an organic light emitting diode OLED driven with a high-level supply voltage VDD for its driving. A cell driver 28-1 drives the organic light emitting diode OLED. The organic light emitting diode OLED has an anode connected to the supply voltage VDD and a cathode connected to the cell driver 28-1.
The cell driver 28-1 includes a switching thin film transistor T1 being turned on by a scanning pulse applied to the scan line SL to switch a data voltage supplied to the data line DL. A capacitor Cst that charges the data voltage supplied via the switching thin film transistor T1. A driving thin film transistor T2 being turned on by a voltage supplied from the switching thin film transistor T1 or the capacitor Cst to drive the organic light emitting diode OLED.
The driving thin film transistor T2 passes a voltage and a current applied, via the organic light emitting diode OLED, to the drain thereof into a ground connected to the source thereof in a state turned on by a data voltage supplied via the switching thin film transistor T1 or a voltage supplied from the capacitor Cst, thereby driving the organic light emitting diode OLED. Brightness of the organic light emitting diode OLED is proportional to a current amount passed through the driving thin film transistor T2 into the ground.
The driving thin film transistor T2 controlling the brightness of the organic light emitting diode OLED has a threshold voltage that is raised by a deterioration caused by a voltage applied to the gate thereof. Alternatively, the threshold voltage is raised by a peripheral high temperature circumstance because it is made from amorphous silicon. If the threshold voltage is raised in this manner, then brightness of the light emitting diode OLED is lowered because a current amount passed through the driving thin film transistor T2 into the ground is reduced in proportion to the raised threshold voltage value.
An organic light emitting diode display device and a driving method thereof that is capable of automatically compensating for a driving voltage of the organic light emitting diode in accordance with a magnitude of feedback voltages from pixels is desired.
In addition, an organic light emitting diode display device and a driving method thereof that is capable of automatically compensating for a driving voltage of the organic light emitting diode reduced thereby preventing a brightness reduction of the organic light emitting diode is desired.